270 ZOOLOGICAL PHILOSOPHY 



function ever more active, and ultimately endows it with the highest 

 energy. 



Since water-born respiration is the least active, let us examine it 

 first. We shall find that water-breathing organs are of two kinds, 

 which again differ as regards activity ; we shall afterwards note the 

 same thing in the case of air-breathing organs. 



Water-breathing organs are divided into water-bearing tracheae 

 and gills, just as air-breathing organs are divided into air-breathing 

 tracheae and lungs. It is indeed quite obvious that water-bearing 

 tracheae are to gills what air-breathing tracheae are to lungs. {Systeme 

 des Animaux sans Vertebres, p. 47.) 



Water-bearing tracheae consist of a certain number of vessels which 

 ramify and spread in the animal's interior, and open on the outside by a 

 number of small tubes which absorb the water : by this means water 

 continually enters by these tubes, undergoes a kind of circulation all 

 through the animal's interior, carries the respiratory influence there,, 

 and appears to issue forth again through the ahmentary cavity. 



These water-bearing tracheae constitute the most imperfect, the least 

 active, and the earliest respiratory organ created by nature ; that 

 moreover which appertains to animals whose organisation is so low 

 that their essential fluid still has no circulation. Striking examples 

 are found in the radiarians, such as the sea-urchins, star-fishes,, 

 jelly-fishes, etc. 



Gills are also a water-bearing organ, which may moreover become 

 accustomed to breathing free air ; but this respiratory organ is always 

 isolated either within or without the animal, and only occurs in animals 

 whose organisation is sufficiently advanced to have a nervous and a 

 circulatory system. 



Trying to find gills in radiarians and worms merely because they 

 breathe water, is like trying to find lungs in insects because they breathe 

 air. The air-breathing tracheae of insects constitute therefore the most 

 imperfect of the air-breathing organs ; they extend throughout all 

 parts of the animal, carrying with them the valuable influence of 

 respiration ; whereas lungs, like gills, are isolated respiratory organs, 

 which at their highest development are more active than any other. 



For the thorough appreciation of the foregoing doctrine, some 

 attention must be given to the two following principles. 



Eespiration, in animals which have no circulation of their essential 

 fluid, is carried out slowly without any perceptible movement, and in a 

 system of organs which is distributed to almost every part of the ani- 

 mal's body. In this type of respiration, the respired fluid itself conveys 

 its influence to the parts ; the animal's essential fluid goes nowhere in 

 advance of it. Such is the respiration of the radiarians and worms,. 



